### Abstract

Breaking wave-induced loads on offshore structures can be extremely severe. The air entrainment mechanism during the breaking process plays a role in the exerted forces which is still poorly understood. More knowledge needs to be gained in this regard, because a large number of constructions will be installed in the near future in intermediate water depth regions (20-40 m) where waves may break under storm conditions.

This thesis deals with the development of a numerical methodology for the reproduction of breaking waves with the related air entrainment mechanism. The methodology was optimized for the prediction of the roller in spilling breakers which is a mixture of entrained air and water that travels with the wave front with approximately the wave celerity.

The approach couples the Eulerianmultiphase model for the entrained air bubbles with a volume-of-fluid method to capture the free surface. The methodology was implemented within a Computational Fluid Dynamics framework through the libraries provided by the open-source CFD package OpenFOAM. The air entrainment was obtained with a formulation which transfers mass from the air above the free surface into the bubbles in water.

The developed CFD solver was strategically validated against four experimental case studies. In the first case, a flow in a bubble column was reproduced in order to test the capability of the Eulerian multiphase model of predicting the bubble motion and the turbulence field of the water phase. In the second case, the implemented Volume-Of-Fluid method was verified in the simulation of regular spilling waves without reproducing the air entrainment. In the third and fourth case, the performance of the formulation for the air entrainment was analysed by simulating a single deep water spilling wave and regular spilling waves. These tests were repeated without entrained bubbles in order to highlight their effects on the flow.

Finally, the CFD solver was employed to investigate the impact of a laboratory spilling wave on a vertical circular cylinder. Several impacts were reproduced for different distances of the cylinder fromthe breaking point. The role of the roller on the exerted forces was evaluated by performing simulations without the inclusion of entrained bubbles. Moreover, the computed in-line forces were compared with the measurements of an experiment conducted at the Technical University of Denmark.

This thesis deals with the development of a numerical methodology for the reproduction of breaking waves with the related air entrainment mechanism. The methodology was optimized for the prediction of the roller in spilling breakers which is a mixture of entrained air and water that travels with the wave front with approximately the wave celerity.

The approach couples the Eulerianmultiphase model for the entrained air bubbles with a volume-of-fluid method to capture the free surface. The methodology was implemented within a Computational Fluid Dynamics framework through the libraries provided by the open-source CFD package OpenFOAM. The air entrainment was obtained with a formulation which transfers mass from the air above the free surface into the bubbles in water.

The developed CFD solver was strategically validated against four experimental case studies. In the first case, a flow in a bubble column was reproduced in order to test the capability of the Eulerian multiphase model of predicting the bubble motion and the turbulence field of the water phase. In the second case, the implemented Volume-Of-Fluid method was verified in the simulation of regular spilling waves without reproducing the air entrainment. In the third and fourth case, the performance of the formulation for the air entrainment was analysed by simulating a single deep water spilling wave and regular spilling waves. These tests were repeated without entrained bubbles in order to highlight their effects on the flow.

Finally, the CFD solver was employed to investigate the impact of a laboratory spilling wave on a vertical circular cylinder. Several impacts were reproduced for different distances of the cylinder fromthe breaking point. The role of the roller on the exerted forces was evaluated by performing simulations without the inclusion of entrained bubbles. Moreover, the computed in-line forces were compared with the measurements of an experiment conducted at the Technical University of Denmark.

Original language | English |
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Place of Publication | Kgs. Lyngby |
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Publisher | Technical University of Denmark |

Number of pages | 161 |

Publication status | Published - 2016 |

### Cite this

Tomaselli, P. D. (2016).

*A methodology for air entrainment in breaking waves and their interaction with a mono-pile*. Kgs. Lyngby: Technical University of Denmark.